The use of tungsten filaments in incandescent lamps is well established. Also well established is the fact that tungsten filaments do not heat to a uniform temperature upon passage of an electric current therethrough. Rather, localized hot spots are generally observed. Such hot spots are the filament life-determining factor in a well-made lamp because of their higher rate of tungsten evaporation. A runaway condition thereby exists, with evaporation-promoted thinning of the wire and consequent increased ohmic heating and ever higher localized temperatures.
Hot spots on a filament may arise because of nonuniform wire cross sectional area, nonuniform cross sectional shape, variations in wire surface smoothness, nonuniformly spaced turns or segments of turns in a coiled or coiled coil geometry, and other reasons. The subsequent coiling and bending processes used in the forming operations, causes abrasions, cracks and splits in the filament which further contribute to the occurrence of hot spots. While good lamp manufacturing practice strives to reduce all such contributing factors, the presence of hot spots is nevertheless ubiquitous.
It is well known to those skilled in the art that a halogen cycle based on flourine or its compounds differs from those based on bromine and the other halogens in that with fluorine evaporated tungsten is redeposited back onto the filament in a rate that increases with filament temperature. This is because of the relatively greater thermal stability of tungsten fluoride as compared to the other tungsten halides. In effect, the fluorine cycle renders the filament much more stable because the hot spots, which are more prone to evaporation, are also significantly more effective in thermally breaking down tungsten fluoride and thereby depositing tungsten back onto the filament. If practice, it is found that, at hot spots, tungsten deposition occurs at a higher rate than does evaporation, and the net effect is for a fluorine-cycle lamp to continually repair its filament.
The attractiveness of the fluorine cycle in incandescent lamps is somewhat offset by the toxicity of fluorine compounds (either initially or after operation of the lamp) and the technical difficulty of providing a lamp vessel and lead wires that are resistant to fluorine attack. U.S. Pat. No. 4,256,988, e.g., addresses the problems of how to protect a lamp envelope and the filament supporting structure from attack by fluorine in a tungsten-fluorine lamp. The suggested method involves coating the interior of the lamp envelope and the internal structure with fluorine resistant compounds.
While this approach is interesting, it would certainly be expensive; and, it does not solve the problems occasioned by leaving in the hands of consumers a vessel loaded with toxic fluorine or fluoride compounds.